Shortest Path Bridging (SPB), as specified in the Institute of Electrical and Electronics Engineers (IEEE) 801.1aq standard, is a computer networking technology intended to simplify the creation and configuration of networks, while enabling multipath routing of data traffic across the networks.
Shortest Path Bridging is the replacement for the older Spanning Tree Protocols (STP) (defined by IEEE 802.1D STP, IEEE 802.1wRSTP, IEEE 802.1s MSTP) that is used to determine paths for forwarding data traffic across segmented networks joined by a set of bridges. This system enabled layer 3 routing of data traffic over a layer 2 network like Ethernet or token ring. STP permitted only a single path toward a root bridge and blocked any redundant paths that could result in a Layer 2 loop. SPB allows all paths to be active with multiple equal cost paths, and provides support for much larger Layer 2 topologies (up to 16 million compared to the traditional 802.1Q WAN limit of 4,096). SPB also supports fast convergence times, and improves the efficiency of the mesh topologies through increased bandwidth and redundancy between all devices, allowing data traffic to be load shared across all paths of a mesh network. To enhance resiliency in the access layer SPB can also be integrated with link aggregation functions, such as standards-based 802.1AX and proprietary multi-chassis link aggregation (MC-LAG) implementations.
The technology provides logical Ethernet networks on native Ethernet infrastructures using a link state protocol (LSP) to advertise both topology and logical network membership. Packets are encapsulated at the edge either in MAC-in-MAC 802.1ah or tagged 802.1Q/802.1ad frames and transported only to other members of the logical network. Unicast, multicast, and broadcast are supported and all routing is on symmetric shortest paths. The control plane is based on the Intermediate System to Intermediate System (IS-IS), leveraging a small number of extensions defined in RFC 6329. For deployments within a Data Center, SPB overall provides good traffic management, when comparing to other technologies for controlling data traffic such as Ethernet virtual private network (VPN), which is well suited for interconnecting Data Centers.